1. Field of the Invention
The present invention is directed to a method as well as to a control apparatus for tracking a contrast agent in an examination subject using a medical imaging device, such as a magnetic resonance imaging apparatus or a computed tomography apparatus, in which the examination subject is disposed.
2. Description of the Prior Art
In examinations involving a contrast agent, it is often required to trigger pickup at that time at which a previously injected volume of contrast agent reaches the examination volume of interest. The contrast agent is typically injected intravenously, and the examination is conducted as soon as the contrast agent is located in certain arteries. For example, in dynamic liver examinations, the liver is recorded during the arterial phase. There are different approaches with respect to this timing, according to the imaging method applied. In the increasingly popular MR angiography involving a contrast agent, care must be taken that at least the center of the k-space is recorded while the contrast agent concentration is high. The physical relations underlying this rule are detailed in U.S. Pat. Nos. 5,417,213; 5,553,619; 5,579,767 and 5,590,654, for example. Of course, for this approach an exact timing becomes more important as the duration of the increased contrast agent concentration becomes shorter relative to the total measuring time for the image acquisition.
In many angiography examinations, particularly in the abdominal region, the image data measurement must ensue in a period of arrested breathing, in order to avoid image artifacts due to respiratory motion. In such examinations, the patient is given the instruction to hold his or her breath prior to the beginning of the measurement. Of course, to make this possible it is necessary to know the starting time of the contrast agent enrichment, and also of the beginning of the image data measurement, in advance.
Previously, this problem was addressed by two different approaches. The timespan between injection and arrival of the contrast agent in a relevant examination volume can be separately determined. To this end, it suffices to inject a very small amount of contrast agent into the patient and to determine the arrival of the contrast agent in the relevant examination region by an imaging method. The time between a injection and arrival, which is termed the circulation time, is measured. The actual examination is then conducted with a normal amount of contrast agent, it being known on the basis of the previously determined circulation time when the image data measuring is to be started. With this knowledge, an instruction for the patient to arrest breathing can be given at the proper time prior to the image data acquisition.
Another method for MR angiography is described in U.S. Pat. No. 5,590,654. In this method data acquisition from the imaging volume is already started prior to the injection of a contrast agent. Subsequent to the injection of the contrast agent, further measurements are performed continually or periodically until the arrival of the contrast agent in the relevant examination region is established on the basis of changes relative to test measurements conducted without contrast agent. The actual image data acquisition is subsequently started. This method only requires an injection with a normal dose, however, with this method a time problem results in MR angiography, particularly if the image data measurement must be performed in an arrested-breathing phase. Low contrast agent volumes (e.g. 10 to 20 ml) are usually used in MR angiography. Given typical injection rates of 2 ml/s, the contrast agent bolus in the blood has a duration of 8 to 15 seconds. Since the instruction to suspend respiration can be given only when the imaging device has reported the arrival of the bolus in the relevant examination region, approximately 4 to 6 seconds of the total available measuring time of 8 to 15 seconds are lost, while the patient acts on the instruction to suspend respiration is converted into action. The remaining measuring time may already be too short for MR angiography. One solution would be to increase the contrast agent volume, in order to extend the available measuring time, however, due to possible side effects and also due to cost, it is desirable to keep the contrast agent volume small.
It is an object of the present invention to provide a method of the type described above wherein a low volume of contrast agent suffices and wherein image data acquisition can be chronologically synchronized, with optimal precision, with the arrival of a contrast agent bolus in the relevant examination region.
This object is inventively achieved in a method and control apparatus for tracking a contrast agent in an examination subject using a medical imaging apparatus allowing viewing of a relevant observation volume of the examination subject, wherein a test bolus of contrast agent is injected followed by injection of a liquid which is free of contrast agent, followed by the injection of a main bolus of contrast agent at a predetermined time interval xcex94ti following the injection of the test bolus. The arrival of the test bolus in the observation volume is determined by periodically obtaining survey images of the relevant observation volume of the subject using the medical imaging apparatus. Subsequent to detection of the arrival of the test bolus, acquisition of imaging data is started at a time interval xcex94tm, with xcex94tm being defined according to the predetermined time interval xcex94ti.
By means of the two-stage injection of the contrast agent at a precisely defined interval, a precise time for the optimal start of the image data acquisition, and also for the instruction, if needed, to suspend respiration, is obtained. Since the available measuring time can be used optimally, a small volume of contrast agent suffices. In an embodiment of the method, image data acquisition is automatically started at the predetermined time interval subsequent to the arrival of the test bolus. While the known methods require a considerable degree of attention from the examining doctor, particularly if an instruction to suspend breathing is necessary, the inventive method can be conducted entirely automatically. This reduces the error rate considerably. With a control apparatus and two infusion pumps, the contrast agent injection can also be automated.